Combined Raman spectrometer/laser-induced breakdown spectrometer for the next ESA mission to Mars

Among the different instruments that have been pre-selected to be on-board the Pasteur payload on ExoMars is the Raman/laser induced breakdown spectroscopy (LIBS) instrument. Raman spectroscopy and LIBS will be integrated into a single instrument sharing many hardware commonalities. An international team under the lead of TNO has been gathered to produce a design concept for a combined Raman spectrometer/LIBS elegant bread-board (EBB). The instrument is based on a specially designed, extremely compact, spectrometer with high resolution over a large wavelength range, suitable for both Raman spectroscopy and LIBS measurements. Low mass, size and power consumption are the main drivers of the instrument's design concept. In this paper, science objectives for the combined instrument are detailed. Background information on Raman spectroscopy and LIBS are presented, focussing on the synergy of these two techniques. In the last section, the instrument concept resulting from the assessment of the feasibility of the combined Raman/LIBS EBB is presented.     

Alkynyl ruthenium colorimetric sensors: optimizing the selectivity toward fluoride anion

We report on the synthesis of alkynyl ruthenium colorimetric sensors whose receptors are constituted by thiazolidinedione, rhodanine, or barbituric heads as recognition centers for anions. As modifications in the charge density at these recognition centers affect the whole molecule, through the alkynyl ligand acting as a communicating wire, the effects of hydrogen-bonding interactions with the anions were observed with the naked eye and monitored by UV-vis absorption spectrometry. The selectivity of the sensors was improved through electronic modifications of the alkynyl ruthenium subunit: the higher the electron density at the receptor head, the higher the selectivity is. TD-DFT calculations rationalize the long-range electronic communication as a main characteristic of the alkynyl ruthenium species and as a key to improve the selectivity of alkynyl ruthenium-based sensors toward anions.     

Thermodynamic stability of boron: the role of defects and zero point motion

Its low weight, high melting point, and large degree of hardness make elemental boron a technologically interesting material. The large number of allotropes, mostly containing over a hundred atoms in the unit cell, and their difficult characterization challenge both experimentalists and theoreticians. Even the ground state of this element is still under discussion. For over 30 years, scientists have attempted to determine the relative stability of alpha- and beta-rhombohedral boron. We use density functional calculations in the generalized gradient approximation to study a broad range of possible beta-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework. The two most stable structures are practically degenerate in energy and semiconducting. One contains the experimental 320 atoms in the hexagonal unit cell, and the other contains 106 atoms in the triclinic unit cell. When populated with the experimental 320 electrons, the 106 atom structure exhibits a band gap of 1.4 eV and an in-gap hole trap at 0.35 eV above the valence band, consistent with known experiments. The total energy of these two structures is 23 meV/B lower than the original 105 atom framework, but it is still 1 meV/B above the alpha phase. Adding zero point energies finally makes the beta phase the ground state of elemental boron by 3 meV/B. At finite temperatures, the difference becomes even larger.     

Reaction rates as a function of scale within ionic liquids: microscale in droplet microreactors versus macroscale reactions in the case of the Grieco three-component condensation reaction

Task-specific ionic liquids (TSILs) and more specifically binary task-specific ionic liquids (BTSILs), a unique subclass, have been shown to be excellent supports for solution-phase chemistry. The negligible volatility of ionic liquids enables their use as stable droplet microreactors in atmospheric environments without oil protection or confinement. These droplets can be moved, merged and mixed by electrowetting on a chip. Solution-phase synthesis can be performed on these open digital microfluidic labs-on-a-chip as illustrated by a study of the Grieco three-component reaction in [tmba][NTf(2)]-droplet (tmba=N-trimethyl-N-butylammonium NTf(2)=bis(trifluoromethylsulfonyl)imide) microreactors. A detailed study of matrices and scale effects on conversion and kinetic rates of this three-component condensation is presented in this paper. Reactions have been shown to be slower in droplets than in batches in the absence of additional mixing. Also, a significant influence of the ionic-liquid matrix has been observed. Finally, an increase of droplet's temperature resulted in a kinetics enhancement so as to reach macroscale reaction rates, probably because of a much better mixing of reaction's components involving a Marangoni's effect.     

Asymmetric synthesis of antimicrotubule biaryl hybrids of allocolchicine and steganacin

The asymmetric synthesis of novel axially chiral biaryl compounds 5 a-f containing a seven- or eight-membered heterocyclic medium ring is described. These molecules can be considered to be structural hybrids of allocolchicine- and steganacin-type natural products. The synthesis featured an atropo-diastereoselective biaryl Suzuki coupling in which a benzylic stereocenter efficiently transferred its stereochemical information to the biaryl axis. The coupling conditions were optimized, and two biphenylphosphane ligands (DavePhos and S-Phos) were found to give the highest yields and diastereoselectivities. A three-element stereochemical model was proposed to explain the observed diastereoselectivities. In a second key step, the medium ring of the target molecules was formed by a stereoselective S(N)1-type cyclodehydration that probably involved a configurationally stable carbocationic intermediate, as supported by calculations. Alternatively, S(N)2-type cyclizations were employed on the same Suzuki coupling products to give the target molecules in a stereodivergent or stereoconvergent manner. These cyclization methods furnished the target hybrid analogues 5 a-f with ee values above 94 %. All analogues were evaluated as antimicrotubule agents and against a panel of cancer-cell lines using colchicine (1) and N-acetylcolchinol (3) as references. Promising activities were found for R,aR-configured compounds 5 a, b and 5 f; in particular, ethyl analogue 5 b showed a twofold antimicrotubule activity relative to colchicine.     

Synthesis, fluorescence, and two-photon absorption of a series of elongated rodlike and banana-shaped quadrupolar fluorophores: a comprehensive study of structure-property relationships

An extensive series of push-push and pull-pull derivatives was prepared from the symmetrical functionalization of an ambivalent core with conjugated rods made from arylene-vinylene or arylene-ethynylene building blocks, bearing different acceptor or donor end-groups. Their absorption and photoluminescence, as well as their two-photon-absorption (TPA) properties in the near infrared (NIR) region, were systematically investigated to derive structure-property relationships and to lay the guidelines for both spectral tuning and amplification of molecular TPA in the target region. Whatever the nature of the core or of the connectors, push-push systems were found to be more efficient than pull-pull systems, and planarization of the core (fluorene versus biphenyl) always leads to an increase in the TPA cross sections. In contrast, increasing the conjugation length as well as replacement of a phenylene moiety by a thienylene moiety in the conjugated rods did not necessarily lead to increased TPA responses. The present study also demonstrated that the topology of the conjugated rods can dramatically influence the TPA properties. This is of particular interest in terms of molecular engineering for specific applications, as both TPA properties and photoluminescence characteristics can be considerably affected. Thus, it becomes possible to optimize the transparency/TPA and fluorescence/TPA efficiency trade-offs for optical limiting in the red-NIR region (700-900 nm) and for two-photon-excited fluorescence (TPEF) microscopy applications, respectively.     

Action of α-cyclodextrin on phospholipid assemblies

Interactions of α-cyclodextrin (α-CD) with dimyristoylphosphatidylcholine (DMPC) and Egg phosphatidylcholine (Egg-PC) were studied (i) by analyzing surface pressure-area isotherms and surface tension of phospholipid monolayers formed at the interface between air and α-CD aqueous solutions and (ii) by X-ray diffraction performed on fully hydrated α-CD/phospholipid binary mixtures. The cyclodextrin molecules strongly interact with the two-dimension phospholipid assembly. Their addition into the aqueous sub-phase leads to the removal of part of the phospholipids from the air-water interface: the higher the α-CD concentration, the higher the phospholipid depletion. This should preferentially involve interactions between cyclodextrin and the phosphatidylcholine head group as α-CD is water-soluble and not surface-active. At the three-dimension level, the bilayer packing of the phospholipid lamellar phase appears not affected by the presence of cyclodextrin as shown by X-ray scattering at small angles whereas wide-angle diffraction patterns reveal the formation of a crystalline phase organized in a pseudo-hexagonal lattice usually characteristic of α-CD dimers. These results point out that α-CD should interact with bilayer-forming phospholipid molecules but likely according to a process that would preserve intact at least a part of the multilamellar assembly.     

Copper-carbene complexes as catalysts in the synthesis of functionalized styrenes and aliphatic alkenes

(NHC)-Cu (NHC = N-heterocyclic carbene) complexes efficiently catalyzed the methylenation of a variety of aliphatic and aromatic aldehydes and ketones in the presence of trimethylsilyldiazomethane, triphenylphosphine, and 2-propanol. The copper catalysts are not only inexpensive compared to rhodium complexes, but they also exhibit better functional group compatibility with aromatic aldehydes and ketones. Indeed very high yields were obtained for the formation of styrenes containing nitro, trifluoromethyl, amino, and ester groups, as well as for pyridine-, pyrrole-, and indole-substituted alkenes.     

Crystal structure and photochemical behavior in solution of the 3'-N-sulfamate analogue of thymidylyl(3'-5')thymidine

The 3'-N-sulfamate analogue of thymidylyl(3'-5')thymidine (TnsoT, 1) exhibits a preference for a C3'-endo conformation in the solution and solid states. Its photochemical behavior in solution is compared to that of its natural counterpart, thymidylyl(3'-5')thymidine (TpT, 2), to get further insight into the significance of the C3'-endo conformation on the photoproduct formation at the single-stranded dinucleotide level. Irradiation at 254 nm of 1 led to the same type of photoproducts as observed with 2. However, 1 was significantly more photoreactive than 2, and accordingly, the initial rate of photoproduct formation was enhanced in accordance with its propensity to base stack compared to 2. The corresponding quantum yields were determined and showed that the enhancement factor (1 compared to 2) is moderate for the cyclobutane pyrimidine dimer (CPD) (1.26) and much higher for the (6-4) photoproduct (1.8). These data strongly suggest that the CPD and (6-4) photoproduct arise from distinct minor stacked conformations.